Method for manufacturing compound semiconductor wafer and compound semiconductor device

ABSTRACT

A method for producing a compound semiconductor wafer used for production of HBT by vapor growth of a sub-collector layer, a collector layer, a base layer and an emitter layer in this turn on a compound semiconductor substrate using MOCVD method wherein the base layer is grown as a p-type compound semiconductor thin film layer containing at least one of Ga, Al and In as a Group III element and As as a Group V element under such growth conditions that the growth rate gives a growth determined by a Group V gas flow rate-feed.

TECHNICAL FIELD

The present invention relates to a method for producing a compoundsemiconductor wafer used for production of hetero junction bi-polartransistors (HBTs), and to a compound semiconductor device.

BACKGROUND ART

Hetero junction bi-polar transistors (HBTs) are bi-polar transistors inwhich an emitter-base junction is a hetero junction using a materialgreater in band gap than a base layer for an emitter layer in order toenhance the emitter injection efficiency, and since HBT is suitable assemiconductor devices used in the frequency area higher than microwaveband, they are expected to be used as semiconductor devices for portabletelephones of the next generation.

The structure of HBT is as follows. In the case of, for example, aGaAs-based HBT, generally an n⁺-GaAs layer (sub-collector layer), ann-GaAs layer (collector layer), a p-GaAs layer (base layer), an n-InGaPlayer (emitter layer) and an n-GaAs layer (sub-emitter layer) are inturn grown as crystal on a semi-insulated GaAs substrate by a MetalOrganic Chemical Vapor Deposition (MOCVD), thereby forming a thin filmcrystal wafer having the above-mentioned layer structure where a pnjunction which is an emitter-base junction is of a structure of a heterojunction, and an HBT is produced using the resulting wafer.

FIG. 7 schematically shows a structure of the conventional generalGaAs-based HBT. In the HBT 100 of FIG. 7, a sub-collector layer 102comprising an n⁺-GaAs layer, a collector layer 103 comprising an n-GaAslayer, a base layer 104 comprising a p-GaAs layer, an emitter layer 105comprising an n-InGaP layer, a sub-emitter layer 106 comprising ann⁺-GaAs layer and an emitter contact layer 107 comprising an n⁺-InGaAslayer are in this turn formed as semiconductor thin film crystal layerson a semi-insulated GaAs substrate 101 by a suitable vapor growth methodsuch as MOCVD method, and a collector electrode 108, a base electrode109 and an emitter electrode 110 are formed on the sub-collector layer102, the base layer 104 and the emitter contact layer 107, respectively.

In an HBT constructed as above, the current gain β is shown byβ=Ic/Ib=(In−Ir)/(Ip+Is+Ir), wherein In denotes an electron injectioncurrent from emitter to base, Ip denotes a hole injection current frombase to emitter, Is denotes an emitter/base interface recombinationcurrent, and Ir denotes a recombination current in the base.

Therefore, it can be seen from the above formula that in order toincrease the current gain β, it is necessary to decrease Ir which is arecombination current in the base. This recombination current in thebase is sensitive to the crystallinity of the base layer, and when thereare many crystal defects in the base layer, the recombination current inthe base increases, resulting in decrease of the current gain β. Thus,in order to improve characteristics of current gain of HBT, it isnecessary to make the base layer have a good crystallinity.

As one of the conventional technologies to attain the above object,JP-A-3-110829 proposes a method for producing a compound semiconductorthin film in which the substrate temperature during the growth of thecompound semiconductor thin film is set in the range of 450–650° C. andthe feeding molar ratio of the raw material of Group V and that of GroupIII is set in the range of 0.3–2.5.

According to the above proposed conventional method, it is disclosedthat the carrier concentration can be controlled to 1×10¹⁸ cm⁻3–1×10²⁰cm⁻³, but the method has a problem that when the feeding molar ratio ofthe raw material of Group V and that of Group III and the growthtemperature are determined, the carrier concentration is determinedthereby and thus it is difficult to control the carrier concentration toa desired value.

DISCLOSURE OF INVENTION

The object of the present invention is to provide a method for producinga compound semiconductor wafer and a compound semiconductor device whichcan solve the above problems in the conventional technologies.

The object of the present invention is to provide a method for producinga compound semiconductor wafer where control of carrier concentrationbecomes possible by externally adding impurities, whereby a base layerof good crystallinity can be formed, and to provide a compoundsemiconductor device using the wafer.

For the purpose of solving the above problems, the present invention isemployed under such conditions as giving a growth determined by a GroupV gas flow rate-feed as the growth conditions of the base layer, andthus the crystallinity of the base layer is improved and the currentgain can be remarkably improved. By setting the V/III ratio in the rangeof 1.0–0.3, the growth of the base layer can be a growth determined by aGroup V gas flow rate-feed.

The V/III ratio here is a feed ratio of the raw material of Group V andthat of Group III during the growth of Groups III–V compoundsemiconductor crystals. In the organic metal vapor growth method, theraw materials are generally fed in the state of gas from gas cylindersor bubblers. The feed rate of gas from a gas cylinder is controlled by aflow rate controlling apparatus such as a mass flow controller locatedon a feeding line, and (gas concentration in cylinder)×(gas flow rate)is an actual flow rate of the raw material. The feed rate of gas from abubbler is controlled by a flow rate controlling apparatus such as amass flow controller located on a feeding line of carrier gas flowed tothe bubbler, and (carrier gas flow rate)×(vapor pressure of raw materialin bubbler)/(inner pressure of bubbler) is an actual flow rate of theraw material. The ratio of feed rate of the raw material of Group V andthat of Group III on the actual flow rates of the raw materials fed bythe above systems is generally called V/III ratio. In thisspecification, the term “V/III ratio” is also used according to thisdefinition.

The first embodiment of the present invention proposes a method forproducing a compound semiconductor wafer used for the production of HBTby vapor growth of a sub-collector layer, a collector layer, a baselayer and an emitter layer in this turn on a compound semiconductorsubstrate using MOCVD method, wherein the base layer is a p-typecompound semiconductor thin film layer containing at least one of Ga, Aland In as a Group III element and as a Group V element and is grownunder such conditions that the growth rate gives a growth determined bya Group V gas flow rate-feed.

By selecting the growth conditions of the base layer as mentioned above,crystallinity of the grown base layer is improved, the recombinationcurrent in the base layer can be decreased, and the current gain of HBTcan be increased.

The second embodiment of the present invention proposes a method forproducing a compound semiconductor wafer according to the invention ofclaim 1, wherein the base layer is grown with the V/III ratio being inthe range of 0.3–1.0.

The third embodiment of the present invention proposes a method forproducing a compound semiconductor wafer according to the invention ofclaim 1 or 2, wherein the adjustment of carrier concentration in thebase layer is controlled by the flow rate of methane halide.

The fourth embodiment of the present invention proposes a method forproducing a compound semiconductor wafer according to the invention ofclaim 1 or 2, wherein the adjustment of carrier concentration in thebase layer is controlled by the flow rate of CBrCl₃.

The fifth embodiment of the present invention proposes a compoundsemiconductor device comprising a sub-collector layer, a collectorlayer, a base layer and an emitter layer formed as thin film crystallayers in this turn on a compound semiconductor substrate by vaporgrowth, wherein the life time of minority carriers in the base layer is200 psec or longer.

The sixth embodiment of the present invention proposes a compoundsemiconductor device containing a hetero junction bi-polar transistorand comprising a sub-collector layer, a collector layer, a base layerand an emitter layer formed as thin film crystal layers in this turn ona compound semiconductor substrate by vapor growth, wherein the ratio ofcurrent gain/base sheet resistance of the hetero junction bi-polartransistor is 0.60 or more.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically shows a layer structure of one example of the thinfilm crystal wafer for HBT which is produced by the method of thepresent invention.

FIG. 2 diagrammatically shows the principal parts of a vapor growthsemiconductor production apparatus used for producing the semiconductorwafer shown in FIG. 1.

FIG. 3 is a graph which shows the relation between V/III ratio andgrowth rate of p-GaAs.

FIG. 4 is a graph which shows the relation between flow rate of CBrCl₃and carrier concentration.

FIG. 5 is a graph which shows the relation between V/III ratio andcurrent gain in the base layer.

FIG. 6 is a graph which shows the relation between current gain and baseresistance in the present invention in comparison with that of aconventional example.

FIG. 7 schematically shows a layer structure of a general GaAs-based HBTin conventional technologies.

BEST MODE FOR CARRYING OUT THE INVENTION

One example of the embodiments according to the present invention willbe explained below referring to the drawings.

FIG. 1 schematically shows a layer structure of one example of the thinfilm crystal wafer for HBT which is produced by the method of thepresent invention. This thin film crystal wafer is a compoundsemiconductor wafer used for producing a GaAs-based HBT, and one exampleof the embodiments of producing the semiconductor wafer having the layerstructure shown in FIG. 1 by the method of the present invention will beexplained below. Accordingly, this example should not be construed aslimiting the method of the present invention to only the production ofthe compound semiconductor wafer having the structure as shown in FIG.1.

The semiconductor wafer 1 shown in FIG. 1 has the following structure.The semiconductor wafer 1 is constructed by in this turn laminating aplurality of semiconductor thin film crystal growth layers on a GaAssubstrate 2 comprising semi-insulated GaAs compound semiconductorcrystals by MOCVD method. The semiconductor wafer 1 will be explainedreferring to FIG. 1. The GaAs substrate 2 comprises a semi-insulatedGaAs (001) layer, and a buffer layer 3 comprising an i-GaAs layer isformed on the GaAs substrate 2.

Next, construction of HBT functional layers 4 formed on the buffer layer3 will be explained. The HBT functional layers 4 include an n⁺-GaAslayer acting as a sub-collector layer 41 and an n⁻-GaAs layer acting asa collector layer 42 which are in this turn formed as semiconductorepitaxial growth crystal layers of a given thickness on the buffer layer3. A p⁺-GaAs layer acting as a base layer 43 is formed on the collectorlayer 42 similarly as a semiconductor epitaxial growth crystal layer,and an n-InGaP layer acting as an emitter layer 44 is formed on the baselayer 43. An n⁻-GaAs layer is formed on the emitter layer 44 as asub-emitter layer 45, and an n⁺-GaAs layer and an n⁺-InGaAs layer asemitter contact layers 46 and 47.

A method for forming the above-mentioned respective layers as epitaxialgrowth semiconductor thin film crystal layers by MOCVD method will beexplained in detail below.

FIG. 2 diagrammatically shows the principal parts of a vapor growthsemiconductor production apparatus 10 used for producing thesemiconductor wafer 1 shown in FIG. 1 by MOCVD method. The vapor growthsemiconductor production apparatus 10 has a reaction vessel 12 to whicha raw material gas from a raw material feeding system (not shown) is fedthrough a raw material feeding line 11, and a susceptor 13 for heatingthe GaAs substrate 2 placed thereon is disposed in the reaction vessel12. In this embodiment, the susceptor 13 has a shape of plyhedra, on thesurface of which a plurality of the GaAs substrates 2 are provided, andthe susceptor 13 has a known construction of being able to be rotated bya rotating apparatus 14. A coil for subjecting the susceptor 13 toradiofrequency induction heating is indicated by reference numeral 15.The GaAs substrates 2 can be heated to a given growth temperature bypassing a heating current through the coil 15 from a heating electricsource 16. By this heating, the raw material gas fed into the bufferlayer 3 through the raw material feeding line 11 is heat decomposed onthe GaAs substrate 2, whereby a desired semiconductor thin film crystalcan be vapor grown on the GaAs substrate 2. The used gas is exteriorlydischarged from an exhaust port 12A and fed to an exhaust gas disposalapparatus.

After the GaAs substrate 2 is placed on the susceptor 13 in the reactionvessel 12, GaAs is grown at 650° C. as buffer layer 3 of about 500 nmusing hydrogen as a carrier gas and arsine and trimethylgallium (TMG) asraw materials. Thereafter, sub-collector layer 41 and collector layer 42are grown on the buffer layer 3 at a growth temperature of 620° C.

Then, a base layer 43 is grown at a growth temperature of 620° C. on thecollector layer 42 using trimethylgallium (TMG) as a raw material ofGroup III, arsine (AsH₃) as a raw material of Group V, and CBrCl₃ as adopant for the formation of a p-type base layer. In this case, the baselayer 43 is grown with the V/III ratio being within 0.3–1.0 so as togive a growth determined by a Group V gas flow rate-feed at the growthof base layer 43. If the V/III ratio is more than 1.0, the growth rateis determined by a Group III gas flow rate-feed, and if the V/III ratiois not more than 1.0, the growth rate is determined by a Group V gasflow rate-feed, and thus, with decrease of the V/III ratio, the growthrate decreases.

FIG. 3 shows a relation between the V/III ratio and the growth rate(a.u.) when the flow rate of the gas of Group III is constant and theflow rate of the gas of Group V is varied, and this FIG. 3 shows thefacts explained above. In the area of the V/III ratio being more than1.0, the growth rate is determined by the flow rate of the gas of GroupIII and hence it is constant. In the area of the V/III ratio being notmore than 1.0, the growth rate is determined by the flow rate of the gasof Group V, and since the flow rate of the gas of Group V decreases withdecrease of the V/III ratio, the growth rate decreases with decrease ofthe V/III ratio. If the V/III ratio is less than 0.3, flatness ofcrystals is deteriorated. Therefore, the use of a V/III ratio of lessthan 0.3 is unreal, and for the above reasons, the V/III ratio ispreferably a suitable value in the range of 1.0–0.3.

After the base layer 43 is formed in this way, an emitter layer 44 and asub-emitter layer 45 are grown on the base layer 43 at a growthtemperature of 620° C., and emitter contact layers 46 and 47 are formedon the sub-emitter layer 45.

In the semiconductor wafer 1, since the base layer 43 constituting theHBT is grown with the V/III ratio being within 0.3–1.0 so as to give agrowth determined by a Group V flow rate-feed, the crystallinity of thebase layer 43 is remarkably improved, and hence the recombinationcurrent in the base layer can be made smaller and the current gain ofHBT can be considerably improved.

In the above embodiment, TMG, namely, a Ga-based raw material is used asthe raw material of Group III, but Al-based raw materials or In-basedraw materials can also be used. The Ga-based raw materials, Al-based rawmaterials and In-based raw materials may be solely used, but they canalso be used in combination of some of them. As the raw materials ofGroup V, in addition to arsine, other suitable raw materials of Group Vcontaining As for growing of the base layer 43 may be used.

Since CBrCl₃ is used as a dopant to dope carbon (C) to form the baselayer 43 of p-type, the doping amount of carbon (C) is adjusted byproperly adjusting the flow rate of CBrCl₃ during the growing of thebase layer 43, whereby the carrier concentration of the base layer 43can be controlled independently of the growing conditions.

When the growth temperature is 620° C. and the V/III ratio is 0.9 or0.7. It is realized that the carrier concentration can be independentlycontrolled in the range of 1.0×10¹⁹ cm⁻3–1.0×10²⁰ cm⁻³ by adjusting theflow rate (sccm) of the carrier gas fed to a CBrCl₃ bubbler at atemperature of the dopant CBrCl₃ of 10° C. as shown in FIG. 4. The samemay be said when the temperature is other than 620° C.

The control of the carrier concentration in the base layer 43 can alsobe similarly carried out by passing methane halide at the time ofgrowing and controlling the flow rate thereof in addition to theadjustment of the flow rate of CBrCl₃. As the methane halide, forexample, CBr₄, CBr₃Cl, CBr₂Cl₂, CCl₄ and the like can be used other thanthose above.

When a semiconductor wafer 1 having the layer construction shown in FIG.1 is produced and HBT is produced using this semiconductor wafer 1 asmentioned above, crystallinity of the base layer 43 is improved andhence an amplification device of great current gain can be produced. Inthis case, it is desirable that the life time of minority carriers inthe base layer 43 is 200 psec or longer. Furthermore, the ratio ofcurrent gain/base sheet resistance is preferably 0.60 or more.

A semiconductor wafer having the structure as shown in FIG. 1 wasproduced and an HBT device was produced using the semiconductor wafer asexplained in the following examples. The emitter size was 100 μm×100 μm.Here, a collector current/base current ratio when a collector current of1 kA/cm² was passed was used as current gain β.

The time-resolved PL measurement was impossible because film thicknessof the base layer was thin in the HBT structure. Therefore, themeasurement was conducted on a sample prepared by laminating a thin filmof p-GaAs at 1 μm under the same conditions as in the production of theHBT base layer.

EXAMPLE 1

The growth conditions of base layer 43 were as follows. Growthtemperature: 620° C.; raw material of Group III: trimethylgallium (TMG);raw material of Group V: arsine (AsH₃); dopant for forming p-type baselayer: CBrCl₃; and V/III ratio: 0.9. Under the above growth conditions,the carrier concentration of the base layer 43 of 3.6×10¹⁹ cm⁻³ wasobtained by adjusting the doping amount of C as a dopant. The currentgain β of the HBT device in this case was measured to obtain 180.Further, a ratio of current gain β/base sheet resistance BRs wasmeasured to obtain 0.60.

EXAMPLE 2

An HBT device was produced under the same conditions as in Example 1,except that the V/III ratio was 0.7, and the current gain β of the HBTdevice was measured to obtain 215. Further, the life time of minoritycarriers in the base layer 43 was measured to obtain 230 psec. The ratioof current gain β/base sheet resistance BRs was measured to obtain 0.70.

COMPARATIVE EXAMPLE

HBT devices for comparison were produced under the same growthconditions as in Example 1, except that the V/III ratio was 1.3, 3.3 or25 which was more than 1.0.

When the V/III ratio was more than 1.0, the current gains β were all150. Furthermore, The ratio of current gain β/base sheet resistance BRswas measured to obtain 0.50. The life time of minority carriers in thecase of the V/III ratio being 25 was measured to obtain 160 psec.

FIG. 5 and FIG. 6 show the results of the measurements. Under suchconditions as giving a growth determined by Group V gas flow rate-feedwhere the V/III ratio was not more than 1.0, the life time of minoritycarriers in the base layer was prolonged because of good crystallinequality. It is considered that for this reason, β was improved.

The V/III ratio, ratio of current gain β/base sheet resistance BRs andlife time τ of minority carriers in Examples 1 and 2 and ComparativeExample are as shown below.

V/III ratio β/BRs τ (ps) Comparative Example 25 0.50 160 Example 1 0.90.60 200 Example 2 0.7 0.70 230

According to the present invention, when the growth conditions of thebase layer are those which give a growth determined by a Group V gasflow rate-feed, crystallinity of the base layer can be improved, lifetime of minority carriers can be prolonged, and, furthermore, currentgain can be markedly improved by using a high ratio of current gainβ/base sheet resistance BRs. Moreover, since the carrier concentrationof the base layer can be controlled independently of the growthconditions, the carrier concentration can be easily controlled to thedesired value.

INDUSTRIAL APPLICABILITY

Devices using the compound semiconductor wafer of the present inventionare used as HBT in the frequency area higher than microwave band.

1. A method for producing a compound semiconductor wafer used forproduction of HBT by vapor growth of a sub-collector layer, a collectorlayer, a base layer and an emitter layer in this order on a compoundsemiconductor substrate using MOCVD method wherein the base layer is ap-type compound semiconductor thin film layer containing at least one ofGa, Al and In as a Group III element and As as a Group V element and isgrown under such conditions that the growth rate gives a growthdetermined by a Group V gas flow rate-feed, wherein the adjustment ofcarrier concentration in the base layer is controlled by the flow rateof methane halide.
 2. A method according to claim 1, wherein the baselayer is grown with the V/III ratio being within the range of 0.3–1.0.3. A method according to claim 1 or 2, wherein the adjustment of carrierconcentration in the base layer is controlled by the flow rate ofCBrCl₃.
 4. A method according to claim 1, wherein the base layer isgrown with the V/III ratio being within the range of 0.7–1.0.